Ahmed Abdi Godane, the leader of Somalia's al-Shabab, was killed in a US air strike(Source: US State Department/Reuters)

According to eye-witness reports the suicide bomber deliberately crashed a vehicle crammed with explosives into a convoy of African Union trucks southwest of the capital on the road towards Afgooye.

One witness, Ali Muhidin, told the AFP news agency: "I saw two minibuses with pools of blood, many civilians were killed. The explosion was enormous."

Al-Shabaab spokesman Abduaziz Abu Muscab told Al Jazeera that the convoy was targeted because of the presence of an American commander who trained Somali special forces.

"We succeeded in killing him and three other Americans," said Abu Muscab. "Also a South African mercenary died in the attack. It was a retaliation for the killing of our leader. This is not going to be the last attack targeting Americans."

It's not known if any Americans were killed in the attack. Most of the dead are believed to have been civilians aboard two buses travelling behind the convoy, which was also carrying Lower Shabelle governor Abdikadir Mohamed Nur who later described the attack.

This is not going to be the last attack targeting Americans

- Al-Shabaab spokesman Abduaziz Abu Muscab

"As we were passing a bus stop at Hawa Abdi area, a small vehicle drove to us, we did not think it could a suicide car bomber. It stopped between two mini buses as we continued. But immediately the car drove toward us and blew off."